Amorphous (noncrystalline) oxide semiconductors for use in TFTs have higher carrier mobility and larger optical band gaps and can be deposited at lower temperatures as compared with the cases of general amorphous silicon (a-Si). Therefore, the amorphous oxide semiconductors are expected to be applied to next-generation displays which are required to have large sizes and high resolutions and to achieve high-speed drive; and to resin substrates which have low thermal stability. As compositions of oxide semiconductors suitable for these applications, for example, In-containing amorphous oxide semiconductors [In—Ga—Zn—O (IGZO) etc.] have been proposed.
For deposition of the above oxide semiconductor (film), a sputtering method has been preferably employed, in which a sputtering target (hereinafter sometimes referred to as “target material”) composed of the same materials as the film is subjected to sputtering. In the sputtering method, for stabilization of properties of thin films as products and for efficient production, for example, it is important to prevent abnormal discharge or the like during sputtering, and various techniques have been proposed.
For example, Patent Document 1 proposes a technique of suppressing abnormal discharge by refining average grain size of grains, for an ITO target.
Moreover, Patent Document 2 proposes a technique of preventing a target material from cracking during sputtering by increasing sintered density and also refining grain size, for an ITO target.
Furthermore, Patent Document 3 proposes a technique of increasing conductivity of a target material, thereby suppressing abnormal discharge during sputtering, by subjecting an In—Zn—O-based composite oxide to an annealing treatment in a reducing atmosphere after sintering.